baby screaming when being put to sleep

So my 6 month old has been sleep trained through the CIO method and she did just fine, it only took a few days and she really didn't cry that long,  but all of the sudden she's been screaming when being put to sleep for no apparent reason. I tried rocking her and soothing her but nothing helps. It's even worse when I lay her down to fuss it out a little or even when she's asleep she will wake back up. I have no idea why she's doing this either. I don't think she's teething? any suggestions?